Microbiology Demystified

from one host to another. Parasitic helminths have a very complex reproductive
system that produces fertilized eggs (zygotes) that infect the host organism.

LIFE CYCLE OF HELMINTHS

The life cycle of parasitic helminths that go through a developmental larval stage
involves an intermediate host.
Dioecious adult helminths are of one sex. That is, one individual has a male
reproductive system and another has a female reproductive system. When these
two adult helminths with different sex organs occupy the same host organism,
sexual reproduction can occur. Monoeciousadult helminths are hermaphroditic
(an organism that has both female and male reproductive organs). Some of
these monoecious helminths can fertilize themselves while others may fertilize
each other.

FLATWORMS

Flatworms, also known as platyhelminths, are mostly parasitic, aquatic organ-
isms that range in size from 1 millimeter to 10 meters, as in the case of a tape-
worm. There are more than 15,000 known species of flatworms. A flatworm has
both male and female reproductive parts (monoecious). Most but not all of their
oxygen and nutrients is absorbed through their body wall.
There are two types of flatworms:

• Flukes:Flukes are flat, leaf-shaped bodies that have an oral and a ventral
  sucker that are used to hang on to the body of a host. Flukes live is inside
  the intestines or on tissues of humans. Three common flukes are:
  
  
  • Schistosoma:This genus of flukes causes the disease schistosomiasis, a
    debilitating disease that causes portal hypertension and liver cirrhosis.
  
  
  • Paragonimus westermani: P. westermani causes paragonimiasis, which
    is the result of the fluke’s depositing eggs into the bronchi of the lung.
  
  
  • Clonorchis sinensis: C. sinensis, also known as Chinese liver fluke,
    causes clonorchiasis, which occurs when the fluke latches inside the
    liver.
  
  
  
  


• Tapeworms: Tapeworms have a knob-like “head,” called a scolex,with
  hooks that allow it to attach to the wall of the intestine of vertebrate ani-
  mals (including humans). Tapeworms have a series of flat, rectangular

CHAPTER 11 The Eukar yotes^179